Devices have long been available for detecting buried pipes or other objects. However, presently available devices have significant drawbacks that prevent their reliably locating the underground object in question with a sufficiently high accuracy required for desired operability, e.g., within one or two centimeters. As well, most prior art devices take considerable time to locate the object. They require many discrete tests or continuous searching so that the search time is typically rather long. As well, these devices tend to have little or no value for locating, for instance, the path of movement of proximate objects on the surface or in air with high accuracy as may desired for many various purposes ranging as widely as from robotic control to analyzing the path of a golf swing.
More specifically, prior art devices do not have sufficiently high underground locating accuracy (within 1 or 2 centimeters) for two dimensional X-Y positioning, e.g., positioning relative to surface landmarks. They also lack or do not provide sufficient accuracy (within 1 or 2 centimeters) for three dimensional X-Y-Z positioning, i.e., positioning that provides depth information as well as relationships to surface landmarks. Furthermore, the accuracy of most or all prior art detectors or methods of detection are very sensitive to depth and rapidly lose accuracy and reliability with increasing depth of the buried object. Variable soil conditions significantly affect the operation of presently available detectors to the extent that the same readings are not consistently obtained if ground conditions change. Presently available devices for locating buried objects can be categorized in roughly five classifications that are discussed below.
Records are often used to locate buried pipes. However, records may be inaccurate or unhelpful for many reasons, some of which are discussed here. For instance, records may be permanently lost, or they may be temporarily lost due to improper filing in a large filing system, or they also become torn, faded, or otherwise unreadable in whole or relevant part. References required by records such as landmarks may be lost, survey errors can arise due to improper or inaccurate measurements or recording, unmarked or inaccurately recorded repairs can invalidate records, new local structures can be built, and numerous other changes may occur with time to cause inaccuracies in records.
Presently available electromagnetic and/or magnetic object locators require the object to be within about three feet of the surface. Otherwise, the signals for detection are so greatly attenuated by the earth that they may be unusable or take considerable time to evaluate. The object to be located with such systems must be metallic and must be sufficiently large to produce an adequate signal. Since the detection must occur within about three feet, the approximate relative position of the object must be known or determinable to avoid significant time delays in locating the object. Even after detection is accomplished, it is often difficult or impossible to know whether the detector instrument is directly over the buried object (X-Y position determination inaccuracy) due to signal variations that are related to factors other than proximity of the object such as shape and orientation of the buried object. Three dimensional accuracy that includes the depth or Z coordinate is typically not available with such systems.
Various types of buried markers may be used to improve the accuracy of the detectors. For instance, magnets may be buried alongside of the object to be detected. However, magnets that are strong enough to be detected without excessive false signals tend to be expensive and have a relatively limited lifetime. Other types of markers have been used, such as inductive wire loops and/or capacitive resonant circuits. However, these markers may require accurate positioning or placement to avoid errors in determining when the detector instrument is directly above the object. Furthermore, the marker inaccuracies due to inexact placement, e.g., horizontal loop positioning, will often increase with increasing depth. As well, the depth of the object must not be too great to cause excessive signal attenuation so that signals become too weak for reliable detection.
Soil conditions that affect the above discussed detectors also complicate operation of active and passive acoustic detectors. For instance, dry unconsolidated sand may cause a loss of signal in acoustic detectors because dry, loose soil conditions are not conducive to efficiently conveying the required level acoustic energy for accurate measurements. Variations in the soil such as harder regions or tree roots or rocks may falsely indicate the detection of the desired object. The relatively close proximity of a target, such as a buried pipe, may also present a significant problem for some types of acoustic imaging and acoustic holography.
Ground probing radar has been used in the past with only limited success. Moisture content of the soil must be relatively low for adequate accuracy. As well, homogeneity of the soil should be high. Even if these two requirements are met, a trained professional operator is necessary. Ground probing radar presently does not work well enough to satisfy the requirements of the natural gas industry.
Because of the long-felt need to provide a means for detecting buried objects, numerous inventors have attempted to provide systems for this purpose. The following patents discuss some of the efforts that have been put forth in this area to solve such problems.
U.S. Pat. No. 4,767,237 issued Aug. 30, 1988, to Cosman et al., discloses a marking tape that is positioned underground and above utility lines. The marking tape carries a pair of closely spaced, parallel insulated conductors that extend along the length of the tape and at least one other insulated conductor extending along the length of the tape which is separated from the pair of conductors at a distance that is greater than the distance between the conductors of the pair of conductors. Passive markers having a resonant circuit can be selectively placed along the tape so that the resonant circuit is electromagnetically coupled to the pair of conductors and the other conductor.
U.S. Pat. No. 4,001,822 issued Jan. 4, 1977, to Fred Sterzer, discloses an electronic license tag or plate formed into a unitary structure and including a single antenna system cooperating in a system comprising a harmonic radiator which transmits a pulse coded identification signal in response to an interrogation signal.
U.S. Pat. No. 4,118 662 issued Oct. 3, 1978, to Harold Weber, discloses a method and apparatus for locating concealed or buried structures. The portable device couples, either by direct or indirect connection, high frequency alternating current energy into a more conductive structure that is usually obscured in a less conductive medium.
U.S. Pat. No. 4,673,932 issued Jun. 16, 1987, to Ekchian et al., discloses a computerized transceiver that repeatedly sweeps through a set of frequencies to interrogate a plurality of groups of items in a shelf. Items in each group are tagged with a printed circuit antenna tuned to frequencies assigned to each group.
U.S. Pat. No. 5,099,227 issued Mar. 24, 1992, to Theodore Geiszler, discloses a proximity detection system that combines the advantages of an electric field coupling mechanism with those of an electromagnetic coupling mechanism to overcome the disadvantages of the respective individual coupling mechanisms. Data is transmitted to the receiver using both an electric field coupling mechanism and an electromagnetic field coupling mechanism. The receiver is provided with a preamplifier circuit that can simultaneously receive signals using either or both of these coupling mechanisms.
U.S. Pat. No. 5,017,415 issued May 21, 1991, to Cosman et al., discloses a marker tape for locating a buried conduit that has a plurality a electronic markers spaced thereon at predetermined intervals. The tape is a nonconductive ribbon and the markers are preferably passive circuits tuned to a specific frequency. The markers may be oriented in a predetermined pattern.
U.S. Pat. No. 4,147,973 issued Apr. 3, 1979, to Harold Weber, discloses a device that accepts first, or higher, frequency electromagnetic signals of various average levels and produces a second, or lower, frequency output, the rate of which varies in relation to the average first frequency level.
U.S. Pat. No. 4,263,552, issued Apr. 21, 1981, to Harold Weber, discloses a portable device that produces an indication of the relative direction of lay for a covert structure. Two signals are produced, one signal for each ear, that provide a "right" or "left" orientation relative to the covert structure.
U.S. Pat. No. 4,866,388, issued Sep. 12, 1989, to Cosman et al., discloses a method and system for determining the location of a first insulated conductor and a plurality of similar branch conductors that are branches of the first conductor. A passive marker having a resonant circuit that includes an inductor and a capacitor is provided for each of the branch conductors and is positioned to be inductively coupled to the first conductor and the associated branch conductor.
U.S. Pat. No. 3,914,762, issued Oct. 21, 1975, to Richard Klensch, discloses an electronic detection and identification system operating with microwave frequencies wherein a transmitted continuously transmits a beam of electromagnetic energy, in a predetermined direction, so as to impinge on an identification tag suitably attached on a passing object. The identification tag then radiates a beam of energy at a harmonic frequency that contains an identification code. The receiver generates signals representative of the identification code.
U.S. Pat. No. 4,255,710, issued Mar. 10, 1981, to Harold Weber, discloses a metal detector that employs a beat frequency oscillator and at least two search oscillators. The search oscillators each include a loop antenna. The two search oscillators produce two signals that are used to provide left-hand and right-hand indication of a buried object.
U.S. Pat. No. 4,314,373, issued Feb. 2, 1982, to Robert Sellers, discloses a passive transmitter that includes a varactor diode. The varactor diode produces a harmonic output when the tuned circuit is energized from a source of electromagnetic radiation.
U.S. Pat. No. 5,438,266, issued Aug. 1, 1995, to Tony H. S. Tsang, discloses a method and system for locating buried conductors, specifically a substation grounding grid. The apparatus indicates location of the conductors as it is passed across the ground surface. The search unit has a vertical axis coil tuned to pick up a signal that is generated from a power unit, applied to the conductors, which signal is interpreted for phase reversal.
U.S. Pat. No. 4,119,908 issued Oct. 10, 1978, to Cosman et al., discloses systems and methods for locating points along an underground conductor wherein tuned passive marker elements are disposed adjacent and alongside the conductor at certain points. A transmitted signal is coupled to the conductor to create a field about the conductor so that a receiver swung back and forth laterally of the conductor will have a peak-null-peak output. However, as a marker is approached, the receiver will have a peak output in the normally null region due to a field generated by the passive marker element to thereby locate the passive marker element.
U.S. Pat. No. 5,045,368, issued Sep. 3, 1991, to Cosman et al., discloses a marked conduit having a plurality of electronic markers spaced thereon at predetermined intervals. The distance between the markers encodes information about the buried conduit. The markers are preferably passive circuits tuned to a specific frequency. The markers may be oriented in a predetermined pattern to provide additional information. The markers may be attached to the outer or inner surface of the conduit, or may be imbedded in the wall of the conduit.
U.S. Pat. No. 4,873,533, issued Oct. 10, 1989, to Tomoyasu Oike, discloses a marker a buried object. The marker includes a passive resonant circuit that becomes inoperative when the initial position of the buried marker is changed. A lead wire or mercury switch opens to break the marker circuit when the initial position of the marker is altered.
U.S. Pat. No. 4,757,315, issued Jul. 12, 1988, to Lichtenberg et al., discloses a system for measuring distance that includes a transmitter and a transponder. The signal transmitted is reflected with the transmitted and reflected signal phases being compared to determine the distance between the transmitter and the transponder. In one embodiment, the transponder generates a harmonic of the transmitted signal.
The prior art devices discussed above do not take into account varying earth conditions and may simply not operate at varying depths of up to several, meters. Such conditions can produce significant signal variations that make location of a buried object unlikely. Even when working under ideal conditions, the prior art does not disclose an accurate means for quickly locating a buried object within centimeters of its relative position.
Furthermore, the prior art also does not provide a suitable device for accurately sensing the position of a proximate but movable object that may or may not be underground. Satellite positioning systems have found great utility for locating the position of an object on the earth. However, such a system requires several satellites positioned appropriately about the earth and still does not readily provide three dimensional data. Also the resolution of such systems is not useful for determining relatively small movements of objects proximate or close to a tracking system and for tracking the same. Directional antennas may also be used to triangulate the position of an object but this information does not determine three dimensional positioning and may take some considerable time to obtain a fix on the object. Such systems may not have a response time suitable for tracking the desired rate of movement due to the need for physically controlling movement of three different antennas. As well, a directional antenna with means for determining distance to an object may be able to locate a position of an object but will not necessarily be able to readily follow an object due to the need for physical movement of the directional antenna and appropriate programming. This is especially true for tracking an object closely proximate to the antenna, as may be desirable with, for example, ergodynamic studies.
Consequently, there remains a need for a method and system to provide the position of buried objects, as well as proximately located non-buried objects, quickly and accurately. Preferably, the device should also be operable for locating the position of the object in mediums that may have variable conductivities and permittivities. The device should also be able to locate objects with high accuracy in the same medium as may be desired for many applications. Those skilled in the art have long sought and will appreciate the present invention that addresses these and other problems.